1. Field of the Invention
This invention relates particularly to an inverter device that drives an AC motor as a mechanical power source for industrial machines of various types in which repetitive operations are carried out in a preset running pattern.
2. Description of the Related Art
FIG. 3 is a circuit diagram showing an exemplary traditional inverter device of this type, that is, so-called V/f-control inverter.
In FIG. 3, 10 represents an inverter main circuit having a configuration shown in FIG. 4, which will be described below, and 20 represents a control circuit that controls the supply of desired AC power to an AC motor 1 from the inverter main circuit 10. The control circuit 20 includes: an acceleration/deceleration adjustor 21 that increases or decreases, with a desired gradient, a frequency designation value from a high-order controller, not shown, for controlling the running operation of the industrial machine, and ultimately converts it to a frequency setting value f1* coincident with the frequency designation value; a voltage pattern generator 22 that generates a primary voltage setting value V1* (DC quantity) of the AC motor 1 in accordance with a boost quantity for increasing the voltage in the V/f control and in accordance with the frequency setting value f1* in order to maintain a constant magnetic flux even when the AC motor 1 is running in a low-speed zone; an integrator 23 that outputs a phase setting value Θ* acquired by time integration of the frequency setting value f1*; a voltage designation value generator 24 that generates three phase AC voltage designation values vU*, vV*, vW* based on the phase setting value Θ* and the primary voltage setting value V1*; a carrier generator 25 that generates a carrier frequency that is gradually changed, if necessary, on the basis of the frequency setting value f1*; and a PWM circuit 26 that performs a Pulse Width Modulation calculation on each of the AC voltage designation values vU*, vV*, vW*, for example, using a triangular wave signal based on the carrier frequency, and generates a driving signal for the inverter main circuit 10 corresponding to the result of the calculation.
FIG. 4 is a circuit diagram showing an exemplary detailed configuration of the inverter main circuit 10 shown in FIG. 3. When an AC power source 2 such as a commercial power source is applied to the inverter main circuit 10 via a contactor 3, the voltage is rectified by a diode rectifying circuit 11 of three-phase bridge connection. This rectified voltage is changed to a substantially smoothed DC voltage by a smoothing capacitor 12. The DC voltage is converted to an AC voltage having a desired amplitude and frequency by an inverter circuit 13 formed by making three-phase bridge connection of an IGBT and an inverse-parallel diode circuit. The IGBT 14 is turned into the ON-state when the voltage between the two ends of the smoothing capacitor 12 has exceeded a predetermined upper limit value because of a regenerative current caused by a braking operation of the AC motor 1. This ON-state causes the regenerative current to flow into a regenerative discharge resistor 15. As a result, the rise in the voltage between the two ends is restrained. For example, see JP-A-7-135731 and JP-A-2003-134839.
In industrial machines of various types in which repetitive operations are carried out in a preset running pattern, using the traditional inverter device shown in FIG. 3 and FIG. 4, a calculated temperature rise value for each constituent element of the inverter main circuit 10 in one cycle of the running pattern is theoretically found, and the electric power capacity of each constituent element is selected so that the calculated value will not exceed a tolerance value.
However, when the industrial machine is actually operated, the temperature rise value of a semiconductor device such as the inverter circuit 13 and the regenerative discharge resistor 15 as the constituent elements of the inverter main circuit 10 in one cycle of the running pattern may differ from the calculated temperature rise value. In the worst case, a protection circuit, not shown, of the inverter device can be overloaded after the running pattern is repeated many times (for example, several hundred times or more) in the industrial machine. This raises a problem that the inverter device will be stopped and the subsequent operation cannot be continued.
Traditionally, in order to solve the above problem, the continuation of the operation of the inverter device is monitored while repeating the running pattern a predetermined number of times. However, this monitoring takes time and labor.